Z-Axis build

I used cheap power drill that i have , it not accuracy and not precision but i can make it more precision and accuracy by process of drilling but some hole will have +- 0.2-0.5mm and how to make it still fit when mounting that was simple. OK how? if need to drill 3mm hole then just drill 3.3mm or 3.5mm hole then 0.3-0.5mm of hole will use for adjust aluminum plate when mounthing.